Semiconductor technology has developed over time to the point where device size is measured in micrometers. Consequently, a single layer of conductive wires may be insufficient to meet the specific demands of miniaturization. In this context, three-dimensional multilayer metal interconnections have been developed to fulfill the demands presented by ever-smaller devices. In multilayer metal interconnection fabrication technology, the properties of the metal interconnections would be influenced by two major factors: (1) RC delay caused by an electrically conductive metal wire and a dielectric layer, and (2) cross talk between electrically conductive metal wires. Furthermore, it is well known that signal transmission speed is inversely proportional to the square of a dielectric constant and is proportional to a dissipation factor. Given the above, developing a material having a low dielectric constant is an important issue nowadays.
Poly(2,6-dimethyl-1,4-phenylene oxide) (hereinafter referred to as PPO) developed by A. S. Hay of U.S. General Electric in 1956 pertains to an engineering plastic and a thermoplastic polymer [1]. PPO has a rigid chemical structure and thus is characterized by its high-Tg (about 210° C.), high tensile strength, rigidity, impact strength, and creep resistance and low coefficient of expansion (CTE: 2.9×10−5 in/° F., −20° F. to 150° F.). In addition, PPO advantageously has good solvent resistance, acid resistance, alkali resistance and low water absorption rate. Since a polymer product is often formed by an injection molding technique, PPO also possesses advantageously low molding shrinkage. Regarding electronic properties, PPO has a low dielectric constant.
To fulfill the demands of high performance and dimensional stability, the resin for a copper foil substrate is required to be thermoset. However, due to the limitation of the inherent chemical structure of PPO, PPO is difficult to cure through self-crosslinking. Therefore, developments and applications of PPO substrates have been limited. Ueda et al. conducted an oxidative coupling reaction to copolymerize 2,6-dimethylphenol and 2-allyl-6-methylphenol such that a PPO having pendant allyl group is produced [2]. The modified PPO becomes a self-curable thermoset polymer. The cured modified PPO still possesses high Tg and low dielectric constant. However, because PPO has a high molecular weight, PPO exhibits relatively high viscosity and poor solubility and impregnation to glass fiber. Therefore, the applications of PPO in the copper foil substrate are limited.
In 2006, Ishii et al. synthesized a telechelic PPE macromonomer (PPE-M) having low molecular weight. The end-capping phenolic group of the macromonomer is reacted with 4-chloromethylstyrene to produce a PPO compound having a vinylbenzene end-capping group. The chemical structure of the resulting vinylbenzyl PPE macromonomer (VB-PPE-M) is shown in Scheme (1) below [3]. In 2007, Peters et al. copolymerized PPE-M, an epoxy resin, and cyanate to improve the properties of a thermoset [4]. In 2011, Peters et al. modified the phenolic end-capping group of PPE-M (or Noryl® SA-90) from SABIC such that an double bond is introduced to the end-capping group of PPE-M [5]. As shown in Scheme 1, when a methacrylate end-capping group is attached to PPE-M, a methacrylated PPE macromonomer (M-PPO-M) shown in Scheme (1) is produced, which has product name NORYL™ Resin SA 9000.

High frequency printed circuit boards have strict requirements in terms of electrical and thermal properties and flame retardance. In particular, such circuit boards need to achieve grade V-0 on a flammability test. Therefore, the PPO material in this regard should be improved so as to fulfill market demands and increase its value. However, PPO is flammable and cannot achieve the UL-94 grade V-0 requirement for an electronic device. Recent research shows that an organophosphorus compound can impart a desired flame retardancy to a polymer. In addition, in comparison with a halogen-based flame retardant, an organophosphorus compound pertains to a solid phase flame retardant, which generates less smoke and toxic gases. In contrast, a conventional additive flame retardant not only degrades the material mechanical properties but also reduces flame retardancy due to the migration or volatilization of the flame retardant molecules. Early studies show that adding a flame retardant can achieve advantageous flame retardancy. For instance, Leu et al. demonstrated adding phosphinated nitrogen-containing flame retardant to PPO [6]. Due to the synergistic effect of nitrogen and phosphorus comprised in a flame retardant, the UL-94 flammability reached grade V-0. In addition, in a PPO and polystyrene (PS) mixture system, although the impact strength of PPO is improved, its flame retardancy is still degraded. Therefore, an additional flame retardant should be added to achieve a desired flame retardancy. In this connection, Takeda et al. studied the flame retardancy of PPO/PS by using different phosphorus-containing aromatic retardants [7]. They found that adding a phosphorus-containing flame retardant can effectively reduce the self-extinguishing time of burning. Therefore, the flame retardancy can be improved. Furthermore, in 2014, Lin et al. discloses a series of phosphinate-derivatives, e.g., 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (hereunder referred to as DOPO). They directly introduced a flame-retardant phosphorus-containing group to the PPO structure so as to replace the additionally added flame retardant [8]. Their results reveal that the UL-94 flammability reaches grade VTM-0 when the phosphorus content is 1% and prove that introducing a phosphorus-containing group to PPO can effectively improve flame retardancy.